1. Field of the Invention
The present invention generally relates to a magnetic head actuator and a magnetic disk device, and more particularly to an actuator for a magnetic head and a magnetic disk device having the actuator in which the magnetic head is moved to a predetermined position by driving the arm on the revolving magnetic disk.
For example, in the magnetic disk device, such as a hard disk drive (HDD), the head slider of the magnetic head is lifted over the surface of the revolving magnetic disk and the magnetic-head element performs reading information from or writing information to the magnetic disk in the lifted state of the head slider. In recent years, in order to increase the recording density of the magnetic disk, reducing the lifting height of the head slider and making the magnetic-head element small have been proposed. In order to accurately position the magnetic-head element to the predetermined track of the magnetic disk whose recording density is thus increased, it is necessary to reduce the vibrations of the arm which is moved over the magnetic disk surface.
2. Description of the Related Art
FIG. 1 shows an example of the conventional magnetic disk device 1. The state of the conventional magnetic disk device 1 in which the top cover is removed is illustrated in FIG. 1.
The magnetic disk device 1 generally comprises the housing 2, the magnetic disk 3, the actuator 4 for the magnetic head (called the head actuator 4), and the voice-coil motor 5.
The housing 2 is provided with the cover which is not illustrated, thereby preventing the inclusion of dirt in the magnetic disk device from the exterior and protecting the respective above-mentioned components 3 and 4 and 5 from damage. Moreover, the magnetic disk 3 is rotated at high speed with a predetermined rotation speed by the spindle motor which is not shown in FIG. 1.
As shown in FIG. 2 and FIG. 3, the head actuator 4 comprises the E-block 14 in which the plurality of actuator arms 6 (in this example, the five actuator arms) are provided. Moreover, the shaft hole 12 in which the shaft 8 embedded in the housing 2 is inserted is formed approximately in the center of the E-block 14. Furthermore, the coil 13 which constitutes part of the voice-coil motor 5 is arranged in the position which is opposite to the position where the actuator arm 6 is arranged with respect to the position of the shaft hole 12.
Each of the actuator arms 6 is a plate-like member which is made of aluminum. The actuator arms 6 are provided on the E-block 14 as mentioned above. The suspension attachment part 11 is provided at the leading edge of each of the actuator arms 6, and the suspension 7 on which the magnetic head is carried is attached to the suspension attachment part 11. Usually, the suspension 7 (which is not shown in FIG. 2 and FIG. 3) is fixed to the suspension attachment part 11 by crimping.
The voice-coil motor 5 comprises the coil 13 which is provided in the head actuator 4, the magnet which generates the magnetic force, and the yoke 5a which is provided for applying the magnetic force to the coil 13, etc.
The magnet is constituted so that the magnetic force generated by the magnet may penetrate the coil 13. By varying the current supplied to the coil 13, the force to cause the head actuator 4 to be rotated around the rotation axis of the shaft 8 is generated. Thereby, the head actuator 4 is rotated around the shaft 8 to move the magnetic head, attached to the leading edge of the actuator arm 6 through the suspension 7, to the predetermined position on the magnetic disk 3.
A description will be given of the actuator arm 6. In order to attain weight reduction of the actuator arm 6, the slot 15 is formed in the actuator arm 6 as the through hole which penetrates the actuator arm 6 in the direction perpendicular to the paper of the figure. Thus, the moment of inertia of the actuator arm 6 can be reduced by forming the slot 15 in the actuator arm 6. For this reason, it is possible to attain high-speed accessing of the head actuator 4 with the slot 15 for weight reduction being formed.
By the way, the actuator arm 6 in which the slot 15 is formed has various oscillation modes in the range from several kHz to several 10 kHz. FIG. 4 is a diagram for explaining the oscillation characteristic of the head actuator in the conventional magnetic disk device of FIG. 1.
As shown in FIG. 4, the damping of the actuator arm 6 to the oscillation modes is very low because the actuator arm is formed by the integral structure of a metallic member. The damping coefficient ζ is on the order of 0.01. In other words, the resonance scale factor at the time of resonance will become about 50 times. This means that a great peak of the resonance occurs at the time of resonance, and the positioning accuracy of the head actuator will deteriorate.
On the other hand, the recording density of the magnetic disk is doubled every year as mentioned above, and the track density (TPI) is increased at an annual rate of 50% or more. Therefore, in order to increase the track density, it is necessary to raise the positioning accuracy.
In the latest magnetic disk device, the track pitch of 0.4 micrometers is attained, and the permissible NRRO (the oscillation accuracy) amounts to 50 nm.
Especially, in the case of the high-speed rotation disk (10,000 rpm, 15,000 rpm), when the air flow generated inside when the magnetic disk 3 is rotated at high speed collides with the actuator arm 6, the aerodynamic vibrations of the actuator arm 6 occur. The state in which the actuator arm 6 is oscillated by the natural vibration during the aerodynamic vibration is shown at the positions (about 7.6 kHz, 12.0 kHz) indicated by the arrows A and B in FIG. 4.
Although the high-speed accessing of the head actuator 4 is intended by forming the slot 15, if aerodynamic vibrations occur on the actuator arm 6 and there is the excitation by the natural vibration of the actuator arm 6, then the positioning accuracy will fall by the vibrations of the actuator arm 6. For this reason, it is very difficult to accurately position the magnetic head at the predetermined position.